Paleomagnetism of Middle Proterozoic diabase sheets from central Arizona

Abstract

Paleomagnetic results from 1090 Ma diabase sheets of the southwestern U.S.A. diabase province exposed in central Arizona yield two distinct remanent magnetizations (herein termed ADn and ADr), in accordance with the findings of previous investigations. Magnetization ADn is well-defined and has an in situ mean direction of D = 283.3°, I = 45.1° (k = 17.7, α95 = 8.7°, n = 17 independent observations). A mean pole, after correction of paleomagnetic site means for a net 5° clockwise rotation of the Colorado Plateau and transition zone, is located at 22.7°N, 179.3°E (K = 21.9, A95 = 7.8°). The second magnetization (ADr) gives an in situ mean direction of D = 161.1°, I = −87.5° (k = 22.2, α95 = 19.9°, n = 4 independent observations) with a poorly defined pole at 37.6°N, 247.6°E (K = 6.5, A95 = 38.9°). Rock magnetic and alternating field and thermal demagnetization characteristics indicate the ADn and ADr magnetizations are both carried by low-Ti titanomagnetite. Both magnetizations are interpreted to be primary thermoremanent magnetizations acquired during emplacement and cooling of the diabase sheets at about 1090 – 1100 Ma. Comparison of the ADn pole and published geochronologic data from the Arizona diabase with the well-dated normal polarity poles of the Keweenawan region indicates that mafic magmatism in the southwestern U.S.A. diabase province and in the midcontinent rift was essentially synchronous.